This invention relates to inverter circuits and more particularly to a circuit for controlling the charge across a capacitor according to the decay of a magnetic field.
There are many systems in the prior art which serve to increase a given potential at an output terminal.
In general, such systems or circuits have been referred to as inverter or converter circuits. Such systems develop a relatively high voltage from a lower voltage source, such as a battery.
The techniques for producing such high voltage outputs have been used in the field of power supplies, photographic flash bulb circuits, pulse laser circuitry, spot welding, and so on.
Essentially, there are many techniques for providing a large voltage from a smaller voltage such as that of a battery.
A number of prior art circuits can be found in a reference entitled "Guidebook for Electronic Circuits" by John Markus, published in 1974 by McGraw Hill, Inc., Chapter 52 entitled "Inverter Circuits". An important application of such circuits is in connection with a portable defibrillator. Such devices are used to provide an electrical pulse of large magnitude for terminating cardiac ventricular fibrillation. In such equipment, the magnitude of the output voltage may be several kilovolts. These devices are portable and battery operated, so that according to the prior art an inverter circuit is used to convert a battery voltage to the required kilovolt level.
A typical circuit used in the prior art includes an oscillator which, powered by the battery, serves to convert the low DC level to an AC signal. The oscillator circuits used typically may comprise a so-called blocking oscillator, astable multivibrator, or some other well known oscillating circuit.
The AC signal output of the oscillator is then applied to a step-up transformer. The turns ratio of this transformer provides an AC signal of considerable voltage at its secondary winding, upon application of the output of the oscillator to the primary winding. The high voltage AC signal at the secondary winding is then conventionally rectified to achieve a DC voltage of large magnitude across a capacitor.
In such circuits, the voltage of the battery is selected to be at least approximately equal to the voltage stored across the capacitor divided by the turns ratio of the transformer.
In order to realize proper operation, a typical prior art circuit of the above type will usually include an impedance to limit the current drawn. This impedance causes a reduction in the charge rate of the capacitor as the voltage across the capacitor increases.
Another disadvantage of such circuits resides in the requirement of a considerable change in battery current due to the charge interval of the capacitor. This factor adversely affects battery life and prevents optimum battery utilizaton.
Also, the size of available batteries necessitates a fairly high transformer turns ratio. Circuit efficiency drops with an increasing turns ration, due to the power loss across the secondary winding of the transformer, so that this factor also is a drawback of the prior art.
Generally speaking, such prior art circuits place stringent requirements on the battery and do not operate reliably as the battery voltage varies or decreases.
Such circuits are not well suited for use in devices such as defibrillators due to the fact that any additional impedance reduces the capacitor charging rate and limits the charging time of the capacitor. Speed of operation is an important aspect of such a device, since the life of a patient is at stake.
Another disadvantage of the prior art resides in the fact that many such circuits require center-tapped transformers, since the circuits exhibit push-pull operation.
Also, most prior art circuits require that the voltage at the secondary winding of the step-up transformer be monitored due to the insertion of the current limiting impedance and due to the fact that current flows simultaneously in both the primary and secondary windings. This monitoring requires high voltage components, due to the high voltage secondary signals.